1. Field of the Invention
The present invention relates to a fixing technology, more particularly, a heat plate and semicircular heating or heat-generating element which are effectively used for fixing a toner image. Also, the present invention relates to a belt type fixing device. Especially, the present invention relates to a heat plate for fixation having high reliability and durability and capable of increasing a temperature at a high rate, used for fixing a toner image in an image forming apparatus such as a copier, printer and so forth, which is based on an electrophotographic system; a heat plate for fixing a toner image having a temperature control property, the heat plate being of a temperature control type by which an upper limit of the temperature provided by a heating resistor layer can be set and controlled; a heat plate for fixing a toner image, the heat plate being of a self-temperature-control type by which a predetermined temperature distribution can be automatically set and controlled in a moment; a heat plate for fixing a toner image, the heat plate being of a complete self-temperature-control type having all the above functions; a semicircular heating member for fixing a toner image, the semicircular heating member being formed into a semicircle and curved by means of press forming; and a belt type fixing device into which the above components are incorporated, the belt type fixing device characterized in that energy can be saved and heating can be quickly conducted. Also, the present invention relates to various image forming apparatuses provided with the belt type fixing device of the present invention.
2. Description of the Related Art
In the electrophotographic system, which is widely used in copiers, printers and printing machines, generally positive or negative uniform electrostatic charges are applied onto a surface of a photoconductive insulator such as a photoreceptor drum. After the completion of this uniform electric charging step, when the photoconductive insulator is irradiated with a light image by various means, the electrostatic charges on the insulator are partially erased so that an electrostatic latent image can be formed. For example, when the photoconductive insulator is irradiated with a laser beams, surface electric charges in a specific portion can be erased. Due to the foregoing, an electrostatic latent image corresponding to image information can be formed on the photoconductive insulator. Next, fine powders of a developing agent, which is called a toner, are attached to the latent image portion on the photoconductive insulator in which electrostatic charges still remain. In this way, the latent image can be visualized. Finally, in order to obtain a print from the thus obtained toner image, the toner image is electrostatically transferred onto a recording medium such as recording paper. Finally, the thus transferred toner image is fused and fixed by applying heat, light or pressure.
As is well known, the fixing process for fixing a toner image, which is a final process of the electrophotographic system, is executed by using various fixing devices. At present, the following two types of fixing devices are commonly used. One is a flash fixing device for fusing and fixing toner by irradiating light such as a flash of light, and the other is a heat roller fixing device for fusing and fixing toner while applying pressure to toner by a heated fixing roller which is also referred to as a heat roller.
In general, in the heat roller fixing device, a heat roller and pressure roller are arranged opposed to each other, and a recording paper having a toner image transferred thereon is made to pass between a pair of rollers. The toner image is thermally fused and fixed onto the recording paper when the toner image is heated and pressurized at the same time. The practically used prior art heat roller comprises a light emitting heating tube such as a halogen lamp housed in a metallic pipe made of aluminum or stainless steel. FIG. 1 is a cross-sectional view showing a heat roller type fixing device composed as described above. The fixing device is provided with a heat roller 61 capable of rotating in the direction of arrow “a” and a pressure roller 62 capable of rotating in the direction of arrow “b”. In the heat roller 61, there is provided a halogen lamp 63. On the other hand, a sheet of recording paper 38, onto the surface of which the toner image 51 has already been transferred in the previous process, is conveyed in the direction of arrow “c” and is inserted into a nip formed between the heat roller 61 and the pressure roller 62. Therefore, when the sheet of recording paper 38 passes through the nip between the heat roller 61 and the pressure roller 62, heat and pressure are simultaneously applied to the sheet of recording paper 38, so that the fused toner image can be fixed onto the surface of the sheet of recording paper 38.
However, the above heat roller utilizes radiation heat so as to heat the sheet of recording paper. Therefore, the following problems may be encountered. The heating efficiency is so low that it takes several minutes to several tens of minutes to heat the sheet of recording paper to a predetermined temperature necessary for thermal fixing, for example, it takes several minutes to several tens of minutes to heat the sheet of recording paper to 160° C. Especially, when a copier, which is in a stopped state out of operation, is turned on, it takes time to start the copier and an operator must wait for a long period of time, which lowers the working efficiency.
Recently, a copier or printer has been linked with another electronic device such as a personal computer. Therefore, if an input signal is transmitted to the personal computer during the waiting time, the system does not proceed at once, because it takes time to raise the heat roller temperature. As a result, the processing rate of the entire system is decreased. That is, no matter how the processing rate of the electronic device may be increased, it is difficult to increase the processing rate of the entire system unless drastic measures are taken for increasing the rate of heating the toner fixing section. In order to solve the above problems, it is conventional that the heat roller is energized and heated even while the entire system is waiting for a successive operation. In this connection, about one hundred million electrophotographic devices are operated all over the world. Therefore, when consideration is given to the fact that all of the electrophotographic devices are energized while they are waiting for a successive operation, it is necessary to take countermeasures for saving energy which is consumed in energizing the heat rollers during the waiting time.
As one countermeasure for saving energy, concerning the heating system for heating the fixing device, instead of the halogen lamp system which is used at present, a direct heating system, induction heating system and others have been developed. Concerning the heating efficiency, when the heating efficiency of the direct heating system is 1, the heating efficiency of the induction heating system is 0.8, and the heating efficiency of the halogen lamp heating system is 0.6. According to a revision of the Law of Saving Energy of April in 1999, competition for saving energy has grown more intense while targeting the conformity to Mode Restriction issued by the Ministry of Trade and Industry in 2000 and also targeting the conformity to the Law of Top Runner in 2006. Equipment to satisfy the above laws on saving energy has already been coming into the market while they use their own techniques to save energy. Digital technique has been widely adopted and color images are processed by every image processing device, and further two or more devices are combined with each other so as to enhance the performance. In order to meet demand for increasing the processing rate, enhancing the image quality and saving energy, belt type fixing devices tend to be adopted.
For example, as shown in FIG. 2, the belt type fixing device includes: a fixing roller (having no heat source therein) 61 capable of rotating in the direction of arrow “a”; a heating unit 65; an endless belt 66 provided between the fixing roller 61 and the heating unit 65; and a pressure roller 62 capable of rotating in the direction of arrow “b”, the pressure roller 62 coming into pressure contact with the fixing roller 61 via the belt 66. The heating unit 65 is formed into a roller arranged in parallel with the fixing roller 61. The heating unit 65 is composed of a cylindrical heat conductive roller 67 and a heating sheet 68 bonded inside the heat conductive roller 67. The heating sheet 68 includes a heating resistor capable of heating when an electric current flows into it. Heat generated in the heating sheet 68 is transmitted to the belt 66 via the heat conductive roller 67 surrounding the heating sheet 68. On the other hand, a sheet of recording paper 38, on the surface of which a toner image 51 is formed in the previous process, is conveyed in the direction of arrow “c” and inserted into the nip formed between the belt 66, which is guided on a surface of the fixing roller 61, and the pressure roller 62. Due to the foregoing, when the sheet of recording paper 38 passes through the nip formed between the rollers, heat and pressure are simultaneously applied to the sheet of recording paper 38. Therefore, the fused toner image can be fixed onto the surface of the sheet of recording paper 38.
When importance is attached to improvement of energy saving properties, the most important matter is how to reduce heat radiation from the endless belt for fixing, the fixing roller and the pressure roller. In order to save energy and reduce the temperature rising time and further in order to reduce the manufacturing cost of the device, the most important issue is how to reduce the heat capacity and the size of the device.
Recently, copiers have made rapid progress as follows. Recent copiers are provided not only with a simple function but also a multiple function, that is, the copiers are composed into compound machines having multiple functions of copying, printing and communicating (function of a facsimile device). Further, the copiers can process not only a monochrome image but also a full color image. In these copiers, various sheets of recording paper, the sizes of which are A5, B5, A4, B4 and A3, are used. When sheets of recording paper of small sizes such as A5 or B5 are frequently used, the surface temperature of a sheet threading portion of the heating member in the sheet threading section is decreased. In contrast, the surface temperature of a portion of the heating member in which the sheet of paper does not thread is increased. Therefore, there is a great difference in the surface temperature distribution. In the case of a conventional fixing device in which a halogen lamp is used, this difference in the surface temperature of the sheet threading portion and that of the portion in which the sheet does not thread is so great that a temperature boundary appears on the sheet of recording paper. Especially in the case of equipment in which processing is conducted at a high processing rate, for example, in the case of a color image forming apparatus, this phenomenon is remarkably caused, and the difference in the surface temperature distribution appears directly on a sheet of recording paper.
In the case of forming a monochrome image, no problems are caused. However, in the case of a fixing device used for forming a color image, there is a demand of having both the thermal fixing function and the full coloring function. In order to meet this demand, it is necessary to raise the temperature to 160° C., which is the fixing temperature, in a temperature raising time of not more than 10 seconds, and it is also necessary for the surface temperature distribution of the heating member to be uniform in a range of fluctuation of ±5% while maintaining a high degree of energy saving. In the age of digital technology, various targets of developing the fixing device are set such as a reduction in electric power consumption, shortening of the temperature rising time and a technique for automatically controlling the surface temperature distribution. Especially in the case of equipment in which color images are processed, in order to obtain images of high quality, it is necessary to quickly raise the temperature and for the distribution of surface temperature to be uniform.